Most transparent objects affect the spatial phase relation of a transmitted wave front of light since different parts of the wave front experience slightly different optical path—lengths. The transparent object thereby performs a spatial phase-modulation on the wave front which is unique for the object. The resulting wave front depends upon the object itself as well as upon the initial spatial phase-distribution over the wave front of the incoming light. Therefore, it is normally only of interest to phase-modulate light from a light source having a well defined spatial phase-distribution, typically light originating from coherent light from a laser. When applying light with well-defined spatial phase-distribution, the output becomes a representation of the spatial differences in the optical properties of the object along the path of the light.
In general, a phase shift δ between two rays of light is given byδ=2πΔΛ/λ0+(φ2−φ1),  (1) where λ0 is the wavelength of the radiation in vacuum, ΔΛ is the difference in optical path length traveled by the light rays, and (φ2−φ1) is the initial phase shift between the rays. According to Equation (1), to induce a spatial phase shift, an object has to change the optical path length Λ for one part of the radiation in relation to Λ for another part of the radiation. The optical path length is given by Λ=nL, where n is the refractive index of the medium and L is the distance traveled through the medium. Thus in order to induce a phase shift, one can either change the refractive index of the medium or the distance traveled in the medium.
Thus, generating a wave front with a specific simple spatial phase-modulation is a conceptual simple (though technically cumbersome) task if one knows how to prepare the object to have a specific refractive constant or thickness along the path of the light. If e.g. the object is a window with varying thickness, two trajectories of equal length but through different parts of the window will experience different optical path lengths, which will induce a phase difference in the transmitted wave front. Such an object will always perform the same specific spatial phase modulation when given the same incoming light.
Generating spatially phase-modulated wave fronts with dynamically controllable phase modulation is a difficult task since one needs an object in which the optical path length can be controlled dynamically. Thereby, the phase relation between different parts of the resulting wave front may be dynamically controlled.
In the prior art, dynamically controllable spatial phase-modulators are known as Phase Only Spatial Light Modulators (POSLM), and typically consist of a matrix of dynamically addressable phase-modulating elements in a transmitting or a reflecting configuration. In order to have POSLMs of practical use, one needs a resolution comparable to modern televisions and monitors, typically in the form of an array of tens of thousands of individually addressable phase modulating elements. Such devices are very delicate and sensitive and are only produced by highly specialized manufacturers around the world. All these issues add to the costs of fabrication, and phase modulators are extremely expensive devices.
In transmitting POSLMs, each phase-modulating element is transparent in order for light to pass through the element. Controlling the thickness or the refractive index of each element may control the optical path length of each element.
For dynamically controllable POSLMs, the addressing electronics between the phase-modulating elements introduces large dead-space giving rise to a residual amplitude modulation in the phase-modulated image. This amplitude modulation is multiplied with phase modulation to give a “zero transmission” pattern in the resulting image as well as noise due to diffraction on the dead space areas. Present transmitting POSLMs have a low fill factor, typically 50%.
Seiko-Epson produces a transmitting liquid crystal SLM (LC-SLM) having a high resolution matrix of transparent liquid crystal elements wherein the relative permittivity of each element can be electrically modulated in order to induce a change Δn in the refractive index and thereby the optical path length of the element. The addressing electronics between the phase-modulating elements introduces a large dead-space giving rise to a residual amplitude modulation in the phase-modulated image as well as a low fill factor (<50%).
Meadowlark produces a parallel-aligned liquid crystal modulator (PAL-SLM) with a high fill factor, but this device has a very low resolution in that it contains only 137 phase-modulating elements.
Reflecting POSLMs are typically simpler to fabricate since here, the phase-modulating elements need not be transparent which allows for any bulk electronics to be positioned on the backside. This also allows a much smaller dead space between the elements. Therefore, reflective POSLMs typically have larger fill factors than transmitting POSLMs. In reflecting POSLMs, the optical path length of each element may be controlled by controlling the “depth” of the reflective surface (stroke) or the refractive index of a material layer above the reflective surface.
In general, the reflection configuration of POSLMs is an optically disadvantageous configuration since it increases the required number of optical components needed to guide the light. If a reflective POSLM is arranged in an on-axis geometry, a beam splitter is needed in the optical set-up, which typically introduces 75% loss (two times 50% loss). The beam splitter may be avoided in an off-axis geometry, which however introduces a number of other disadvantages.
Hamamatsu Photonics produces a dynamically controllable PAL-SLM with XGA resolution. Texas Instruments produces a Digital Mirror Device (DMD) having an array of mirrors each of which can be tilted between two positions.
In summary, all known POSLMs are based on the principle of phase modulating the image on a “pixel-by-pixel basis” where the optical path length in each phase modulating element must be dynamically controlled. This is a strenuous, and expensive method and the devices are delicate and sensitive.